Evaluation of Hematologic Toxicity of Stereotactic Body Radiation Therapy (SBRT) to Spinal Metastases

Abstract

Many patients with malignancy develop symptomatic spinal metastases, and radiation therapy is an effective modality for local control and palliation. Bone marrow suppression is a known adverse effect of systemic radiation. However, the effect of focal radiation to spinal metastases on patients’ hematological cell counts, particularly in the setting concurrent myelosuppressive systemic therapy, remains unclear. We aim to clarify the association of stereotactic radiation therapy (SBRT) to spinal metastases and hematological toxicity. A single-institution retrospective analysis of the electronic medical record was performed on patients who were treated with SBRT to solid tumor spinal metastasis between 2011 and 2018. Data was collected on patient demographic, disease, and treatment characteristics. Paired t-test was used to assess change in hematologic lab values at 6 weeks pre and post radiation (p<0.05). Linear regression was used to quantify the magnitude of the association between the change in hematological values and the clinical and treatment covariates. Fifty-three consecutive patient treated with SBRT had complete clinical and radiation treatment data. The median age at the time of SBRT was 62 years (range: 33-79). Histologies included breast cancer (n = 19), renal cell carcinoma (n = 9), melanoma (n = 6), and non-small cell lung cancer (n = 5). Treatment doses were 16-20 Gy in 1 fraction (n = 20), 24-27 Gy in 3 fractions (n = 11), and 30-35 Gy in 5 fractions (n = 15), which corresponded to a median BED10 of 48 Gy (range: 33.6-60). Radiation was delivered to median PTV of 50.4 cc (7.6-1097.8) and 12 patients received myelosuppressive systemic therapy at the time of SBRT. In this cohort, there was a statically significant decrease in the average hemoglobin, white blood cell count, and platelet count of 0.46 g/dL (12.3 -> 11.8, p = 0.016), 0.83 cells/nanoliter (7.1 -> 6.2, p = 0.050), and 38.6 cells/nanoliter (251.0 -> 212.3, p<0.001) before and after RT respectively. No patient experienced clinically significant bleeding, developed a new infection, or was administered a transfusion or a hematologic growth factor during the studied intervals. Neither age, concurrent chemotherapy, PTV size, nor BED10 appeared to be confounders or predictors of these hematologic markers. SBRT is associated with a mild statistically significant reduction in patients’ average hemoglobin, white blood cell count, and platelet count within 6 weeks after treatment. However, it did not result in any clinically significant adverse hematologic event, even in the setting of concurrent myelosuppressive chemotherapy. Further studies are needed to assess the volume of bony disease that can be treated without significant hematologic toxicity.